The coaxial cables commonly used today for transmission of RF signals, such as television signals for example, comprise a core containing an inner conductor and dielectric, and a metallic sheath surrounding the core and serving as an outer conductor. The dielectric surrounds the inner conductor and electrically insulates it from the surrounding metallic sheath. In some types of coaxial cables, air is used as the dielectric material, and electrically insulating spacers are provided at spaced locations throughout the length of the cable for holding the inner conductor coaxially within the surrounding sheath. In other known coaxial cable constructions, an expanded foam dielectric material surrounds the inner conductor and fills the space between the inner conductor and the surrounding metallic sheath.
In order to provide flexibility, some of the coaxial cables of the prior art have used a flexible metallic braid or a thin overlapping flexible metallic foil wrap as the sheath or outer conductor, as disclosed for example in U.S. Pat. Nos. 3,032,604; 3,315,025; 3,662,090 and 3,727,247. However, a disadvantage of this type of overlapping construction is that the discontinuous outer conductor or sheath does not totally shield the cable electrically and the sheath also permits moisture or other contaminants to enter the cable. These conditions of electrical field radiation and moisture ingress are further aggravated by flexure.
A very important function of the metallic sheath in a coaxial cable is to electrically shield the cable from external fields which might interfere with the electrical signal being carried by the cable and also to prevent leakage of the RF signal from the cable. Another important function of the sheath is to seal the cable against the permeation of moisture, which adversely affects the insulating properties of the dielectric and permits corrosion of the inner conductor. Consequently, the metallic sheath used in the majority of the prior coaxial cables is formed from a continuous tube of non-overlapping electrically conductive metal, such as aluminum. Particular efforts have been made in the production of these coaxial cables to ensure that the tube which forms the metallic sheath be both mechanically and electrically continuous. By "mechanically continuous," it is meant that the outer conductor is continuous in both its longitudinal and circumferential extent and mechanically seals the cable against ingress of contaminants such as moisture. This can be measured by measurement of its uniformity of physical properties. By "electrically continuous," it is meant that the outer conductor or sheath is electrically conductive throughout its longitudinal and circumferential extent and seals the cable against leakage of RF radiation either in or out. This can be measured by measurement of the uniformity of electric and magnetic fields external to the cable. In the coaxial cables of known construction, tubular metallic sheaths of a mechanically and electrically continuous construction are produced by various methods, such as by forming a metallic strip or tape longitudinally into a tubular configuration and welding the same, or by extrusion of a seamless metal tube of finite length.
While cables having an electrically and mechanically continuous tubular sheath provide better protection against outside environmental and electrical influences than the prior cable designs noted earlier which use metallic braids and/or foils, the continuous tubular sheath gives the cable significantly less flexibility, and thus makes handling and installation of the cables more difficult. Some improvement in bending properties can be achieved by corrugating the sheath, but the improvement in performance marginally justifies the expense. The cost of the cable is increased and the corrugations reduce the effective electrical diameter and thus adversely affect attenuation.
One of the design criteria which must be considered in producing any coaxial cable is that the cable must have sufficient compressive strength to permit bending and to withstand the general abuse encountered during normal handling and installation. For example, installation of the coaxial cable generally requires passing the cable around one or more rollers as the cable is strung on utility poles. Any buckling, flattening or collapsing of the tubular metallic sheath which might occur during such installation has serious adverse consequences on the electrical characteristics of the cable, and may even render the cable unusable. Such buckling, flattening or collapsing also destroys the mechanical integrity of the cable and introduces the possibility of leakage or contamination.
Bending or buckling of the sheath is particularly troublesome for coaxial cables of the air dielectric type, which, due to the use of spaced discs or spacers, do not exhibit uniform compressive stiffness along their length. These cables are highly susceptible to bending midway between adjacent spacers where the tube is unsupported and the ratio of core stiffness to tube stiffness is at a minimum. However, this problem is no less serious in coaxial cables of the type which use a foam dielectric.
In order to provide adequate compressive strength to withstand the abuse encountered during installation and to prevent buckling, one approach which has been taken in the design of the prior coaxial cables has been to increase the compressive strength of the continuous tubular sheath by providing a relatively heavy wall thickness, typically greater than about 0.025 inches and ranging upwards of 0.055 inches for one inch diameter cables. However, significant loss of flexibility results. Other methods to improve flexibility involve the addition of dielectric, either by placing larger numbers of spacers or by increasing the density of the foam dielectric. This does provide improvement in flexibility, but always at the expense of increased attenuation.